Isocyanate is an organic compound having at least one isocyanate group (—NCO), and is widely used industrially as a raw material for polyurethane, polyurea, etc.
Conventionally, isocyanate has been industrially produced by reaction between amine and phosgene (phosgene method). However, phosgene has various problems such as being highly toxic and troublesome in handling, and requiring careful attention to corrosion of the equipment because it by-produces a large amount of hydrochloric acid. Accordingly, there is a need to develop industrial production methods for isocyanate that can replace the phosgene method.
Known isocyanate production methods that do not use phosgene include a method (urea method) in which amine, urea and/or N-unsubstituted carbamic acid ester, and alcohol are allowed to react (carbamate-forming reaction), and thereafter the obtained carbamate is thermally decomposed.
Meanwhile, it has been known that the carbamate-forming reaction in the urea method by-produces ammonia and carbon dioxide.
Ammonia and carbon dioxide can be used, for example, for production of urea, a raw material component of carbamate-forming reaction, and therefore industrially, it is desired to recover the by-produced ammonia and carbon dioxide, and effectively utilize them.
As a method for producing carbamate in which ammonia is recovered, for example, Patent Document 1 below has proposed a method in which a drained gas containing an organic component, carbon dioxide, and ammonia is washed with an alkaline solution (e.g., caustic soda solution, etc.) to remove organic components and carbon dioxide, and at the same time, to distill ammonia as an overhead product (for example, see Patent Document 1 below).